FAQ: iPhone 5 and 5GHz Wi-Fi

Is iPhone 5 the first smartphone to support 5GHz?

No. A few Android-based rivals, such as the HTC One X and the Samsung Galaxy S III, offer dual-band Wi-Fi. A growing number of new phones will have 5GHz support.

What is 5GHz Wi-Fi?

5GHz is the "other frequency" that Wi-Fi client radios can use, besides 2.4GHz, to connect to an access point or hotspot.

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Why would I want to use it?

First, because the 2.4GHz band is crowded, and therefore has a greater chance of interference. There are lots of devices are using it: Lots of other Wi-Fi devices, including embedded Wi-Fi radios, but also non-Wi-Fi radios like Bluetooth, cordless phones, baby monitors, and stuff like microwave ovens.

Client Wi-Fi radios that support only 802.11g can only connect on 2.4GHz. 802.11a, which so far has not been widely used, runs on 5GHz. 802.11n, which has much higher data rates, can run on either band but most smartphones today that have 11n, such as iPhone 4 and 4S, only run it on 2.4GHz.

And even when the client radio could run on either band, a lot of them today still "decide" to connect on 2.4GHz and stick with it, even when a better connection is available. Vendors and IEEE are working on various technologies to address this, including shifting more control over the connection, and optimizing it to the access point/network, instead of the client.

Second, the 2.4GHz band has only three non-overlapping, 20MHz-wide channels: In crowded environments -- lots of access points, lots of clients -- that can make it hard to get a channel connection. And fewer channels means lower aggregate capacity on the network side.

So what's different about 5GHz?

By contrast, the 5GHz band has, for now, many fewer Wi-Fi clients, and 23 20MHz-wide, non-overlapping channels.

In the 802.11n radio standard, one way that data rates increase dramatically is by combining (or "bonding") two of these channels into a wider 40MHz wireless "pipe." In 2.4GHz, you only have the three channels, and can create only one 40MHz channel.

Apple says that iPhone 5 will deliver a maximum 150Mbps data rate. This would mean using 802.11n, with a single data stream, and a 40MHz channel. Actual throughput will be much less.

How does 5GHz affect network capacity?

"5GHz support on an iPhone is a wonderful thing for Wi-Fi," says William Kish, CTO and co-founder of Ruckus Wireless. "It increases aggregate capacity in challenging environments by something like a factor of 10-12 compared to 11n on 2.4GHz."

Kish defines aggregate capacity as "the total capacity available to all simultaneous users across all of the APs in a given area."

"The higher aggregate capacity is mostly a function of the much larger amount of bandwidth (e.g., the [much greater] number of channels) available in the 5GHz band as well as the more capacity-favorable propagation characteristics of the 5GHz spectrum," Kish says.

For example, an 802.11g network offers 54Mbps of capacity on each of three 2.4GHz channels, for a total capacity of 162Mbps. 802.11a offers the same 54Mbps, but in theory its capacity is much larger due to the larger number of 5GHz channels.

For 802.11n, the numbers are even greater: 150Mbps per channel, for 450Mbps in the 2.4GHz band, and 3.45Gbps in 5GHz. In all cases the actual throughput users get is much less.

What does that mean for throughput?

Kish was recently at the Time Warner Cable Arena in Charlotte, N.C., where a Ruckus Wi-Fi network had been installed, for a live event. He speed-tested his Samsung Galaxy S III smartphone in the network: 60.33Mbps download, and 58.78Mbps upload.

"60Mbps is a serious amount of throughput to a mobile device under real-world conditions!" he says. "I have my fingers crossed that the iPhone 5 delivers similar results."

Are there any drawback to 5GHz?

At the same power level, a 5GHz signal has a shorter wavelength than a 2.4GHz signal. That means it propagates shorter distances. From a client perspective, for example using a Starbucks hotspot or a well-designed hotzone or enterprise WLAN planned for 5GHz, users often may not be affected by either of those characteristics.

Higher-gain antennas and/or increased transmit power (though this is regulated by the Federal Communications Commission) can offset the 5GHz propagation.

Another issue is how the phone will decide which band to use: Will it be set (or settable) to "prefer" 5GHz over 2.4GHz, so the choice is automatic? Or will you be prompted for your preference? Or do you have to manually select one or other?

A June 2012 blog post at WLAN vendor Aruba Networks noted that the vendor's testing of 5GHz mobile devices found "that handover performance [from one access point to another] for a fast-moving device is not quite as good when both bands are enabled as it was for 2.4GHz only ... this is probably due to the larger number of channels that must be scanned. The chip vendors will need to tweak their probing and selection algorithms to tighten this up, and there are some new standard features coming that will help. But this is a minor concern."

What Wi-Fi chip is in the iPhone 5?

We don't know exactly, yet, until the phone is released and it gets the "tear-down" treatment. But historically, Apple has sourced the radio from Broadcom. The iPhone 4S uses the Broadcom BCM4330 -- at the time last year Broadcom's newest WLAN -- Bluetooth, and FM combo chip. It was also used in the Samsung Galaxy S II and, according to Kish, in the newer S III.

That chip supports both bands, but only the 2.4GHz band has been "turned on" in the past in most smartphones.

Can Apple make any other tweaks to boost Wi-Fi performance?

In their in-depth review last year of iPhone 4S, Anand Lal Shimpi and Brian Klug, of AnandTech.com, found the 4S Wi-Fi/Bluetooth antenna, repositioned to the top corner of the phone, had "slightly better received signal strength ... compared to a [iPhone] 4 side by side, and upon checking the FCC documents learned the 4S' WLAN antenna has a peak gain of -1.5 dBi compared to -1.89 dBi on the 4, making it better than the previous model." The higher gain, loosely understood as sensitivity, improves the signal strength and helps sustain the higher throughput connection.

For iPhone 5, Apple has added two glass "RF windows" on the back, at top and bottom. There might also be antenna improvements. Together, these could improve signal strength for the phone.

The more powerful A6 CPU, Apple's system-on-chip or SoC, in the iPhone 5 may also have an impact. In their review of the 4S, the authors noted: "I'm starting to think that the bigger boost is actually thanks in part to a faster SoC." Apple says the A6 delivers twice the performance for new phone as for the 4S.

John Cox covers wireless networking and mobile computing for Network World. Twitter: http://twitter.com/johnwcoxnww Email: john_cox@nww.com Blog RSS feed: http://www.networkworld.com/community/blog/2989/feed

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